A magnetic memory that uses a spontaneous magnetization (herein after, to be merely referred to as “magnetization”) of a ferromagnetic to store a data is one of nonvolatile memory devices to which the greatest attention has been paid in recent years. As a memory cell of the magnetic memory, for example, as disclosed in U.S. Pat. No. 5,650,958, a magnetic resistance element composed of two ferromagnetic layers is used which are separated by a non-magnetic layer of an conductor or insulator. One of the ferromagnetic layers of the two layers is designed such that the orientation of the magnetization is easily changed by an external magnetic field, and the other is designed such that the orientation of the magnetization is not easily changed. The former is often referred to as a magnetization free layer, and the latter is referred to as a magnetization pinned layer. The memory cell stores a digital data as a relative angle between the orientations of the magnetizations of the two ferromagnetic layers. The stored data is held for an extremely long period unless being intentionally rewritten.
When the data stored in the magnetic memory is read, a phenomenon that a resistance of the magnetic resistance element is based on the relative angle between the orientations of the magnetizations of the two ferromagnetic layers, a tunneling magneto-resistance (TMR) effect and a giant magneto-resistance (TMR) are used. The tunneling magneto-resistance effect is used when an insulating film is used as a non-magnetic layer, and the giant magneto-resistance effect is used when a conductive film is used as the non-magnetic layer.
On the other hand, the data write into the magnetic memory is performed such that a write current is sent to a wiring located near the memory cell, and a magnetic field exceeding a switching field is generated, and the orientation of the magnetization of the magnetization free layer is inverted to a desirable orientation by the magnetic field. The wiring used in the data write is often referred to as a word lines a bit line, and a digit line.
Most typically, the magnetization of the magnetization free layer is inverted to the desirable orientation by sending the write current to the orthogonal two write wirings. When the write current is sent to the two write wirings, a synthesis magnetic field is generated in a direction of 45° with respect to the easiness axis of the magnetization free layer. The magnetization of the magnetization free layer is inverted to the desirable orientation by the synthesis magnetic field.
It is possible to perform a data write by selectively flowing the write current to only a selection cell by a transistor, namely, by using one write wiring to invert the magnetization to the desirable orientation. In this case, the write wiring is preferably arranged to have the angle of 45° with respect to the easy axis of the magnetization free layer. This is because the write magnetic field becomes minimal. When the write current is supplied to the write wiring, magnetic field is generated in the direction of 45° with respect to the easy axis of the magnetization free layer. The magnetization of the magnetization free layer is inverted to the desirable orientation by this magnetic field. Hereinafter, this data write using the foregoing method is also referred to as one-axis write. Similarly, it is possible to cause a magnetization reversion by a spin torque by directly supplying a spin current to the magnetization free layer of the selection cell by a transistor. This is also referred to as a spin injection write. The spin injection write is high in cell selection property, similarly to the one-axis write.
In addition, as disclosed in U.S. Pat. No. 6,545,906, when the magnetization free layer has a laminated feri structure (namely, a structure having a plurality of ferromagnetic layers separated by a non-magnetic layer), the direction of the magnetic field applied to the magnetization free layer is rotated inside a plane so that the magnetization of the ferromagnetic layer of the magnetization free layer can be rotated in the desirable orientation. Specifically, the two orthogonal write wirings are extensively arranged such that both have the angles of 45° with respect to the easy axis of the magnetization free layer. Hereinafter, one of the two write wirings is described as a word line, and the other is described as a bit line. In the data write, the write current is first supplied to the word line, to generate the magnetic field in the direction orthogonal to the word line. In succession, the write current is sent to the bit line in a state that the write current is supplied to the word line. Consequently, a magnetic field is generated in a direction oblique to each of the word line and the bit line, typically, in the direction of the angle of 45° with respect to the word line and the bit line. Moreover, in succession, the supply of the write current to the word line is stopped in a state that the write current is supplied to the bit line. Thus, the magnetic field is generated in the direction orthogonal to the bit line (namely, a direction parallel to the word line). In the above process, the write currents are supplied to the word line and the bit line, so that the magnetic field applied to the magnetization free layer is rotated, which can rotate the magnetization of the ferromagnetic layer of the magnetization free layer by 180°. The above data write is also referred to as a toggle write hereinafter.
One of the severest problems in the magnetic memory is a variation in magnetic field for reversion of magnetization of the magnetization free layer, i.e., a switching field. When the variation in the switching field of the magnetization free layer is great, the cells are grouped into write enable memory cells and write disable memory cells in a particular write current. This is inconvenient for the operation of the magnetic memory.
In particular, when the data write is performed by supplying the write current to the two write wirings orthogonal to each other, the variation in the switching field of the magnetization free layer is important. The variation in the switching field of the magnetization free layer decreases a margin of the write current and may bring about a situation that the data is erroneously written to the non-selected memory cell. Also, in the method in which the transistor is used to select the cell, the increase in the variation in the switching field causes the increase in the write current.
One method to suppress the variation in the switching field of the magnetization free layer is to form the magnetization free layer long, namely, make an aspect ratio of the magnetization free layer high. Here, the aspect ratio is a value defined as d/W when a length in a longitudinal direction of the magnetization free layer is assumed to be d and a length in a width direction orthogonal to the longitudinal direction is assumed to be W. Since the aspect ratio is made high, the shape magnetic anisotropy of the magnetization free layer can be increased, to suppress the variation in the switching field of the magnetization free layer.
However, the increase in the aspect ratio of the magnetization free layer is not preferable because the size of the memory cell of the magnetic memory is increased. In particular, in the spin injection write, the decrease in the area of the magnetization free layer is important because this increases the write current density and reduces the write current. In order to make the memory cell of the magnetic memory small, the aspect ratio of the magnetization free layer is desired to be 2.0 or less. However, a method of suppressing a variation in the switching field of the magnetization free layer when the aspect ratio of the magnetization free layer is 2.0 or less is not known.